The present invention relates to equipment used on aircraft to derive conditioned compressed air from a multi-engine power source to other on-board systems requiring a continuous supply of air, such as environmental control systems (xe2x80x9cECSxe2x80x9d), and more particularly to a system and method of controlling bleed air supplied by the engines in order to ensure an equalized supply of bleed air from each of the engines on the aircraft, thereby achieving balanced flow extraction.
Most aircraft utilizing turbine engine propulsion units, both commercial and military, are powered by two or more turbine engines. Virtually all such aircraft divert bleed air from the engines to supply various other systems, most notably the ECS which requires a supply of conditioned air to be utilized for the crew, passengers and electronic equipment environment. In some instances the aircraft de-icing system also uses a portion of the conditioned bleed air.
It has been recognized for some time that in order to more efficiently operate a multi-engine aircraft, it is desirable to extract bleed air from all of the engines equally rather than from only one power plant. If, for example, the entire supply of bleed air to the aircraft is supplied by one engine, the result deteriorates the overall fuel economy, as well as increases wear to the engine supplying the bleed air, since that engine also has to carry its share of the aircraft propulsion duties.
Some of such systems have been disclosed in U.S. Patents. For example, one U.S. Pat. No. 5,155,991, entitled xe2x80x9cBLEED AIR FLOW REGULATORS WITH FLOW BALANCE,xe2x80x9d issued to Bruun, discloses a bleed air flow sharing technique for a two engine system, that uses a venturi and a pressure sensor to estimate the bleed flow in each engine flow path. The differences in the two flow signals is then conditioned to drive the pressure regulator in each engine bleed flow path.
Another two U.S. Pat. Nos. 4,779,644 and 4,765,131, both entitled xe2x80x9cAIRCRAFT ENGINE BLEED AIR FLOW BALANCING TECHNIQUE,xe2x80x9d issued to Benson, disclose a bleed flow control method for each engine using a pressure regulator upstream of a heat exchanger. Since the bleed air pressure drop across the heat exchanger is a function of the flow rate, the pressure drop is used as the feedback signal to control the flow rate.
Yet another U.S. Pat. No. 5,934,614, entitled xe2x80x9cCLOSED LOOP CONTROL SYSTEM FOR CONTROLLING AN AIR DISCHARGE OUT OF AN AIRCRAFT BODY,xe2x80x9d issued to Mueller et al., discloses a fault tolerant control scheme to control multiple air outflow valves for aircraft cabin pressure controls. The xe2x80x9cmasterxe2x80x9d in this system refers to a controller which can send a control signal to the valve controller, termed as xe2x80x9cslave controllerxe2x80x9d, to command the valve. In this case, the slave controller can also send control information (e.g., cabin pressure value, etc.) back to the master controller to enable the master controller to achieve redundant and fault tolerant controls. This system, however, does not address the bleed flow control or flow sharing among engines at all.
Despite all the efforts, balancing bleed flow extraction by the conventional systems has not been entirely satisfactory. The most noticeable trade-off has been diminished fuel economy, since the engine supplying significantly more bleed air will burn more fuel than the other engine saves. Such a reduction in fuel economy will have economic results, particularly in the case of commercial aircraft.
A second, and even more expensive, trade-off is an increased level of engine distress. The engine required to supply substantially more bleed air will tend to wear out faster, since the engine will be running hotter, to compensate for the increased amount of bleed air tapped off. This results in the requirement that the engine be overhauled or replaced at an earlier time, resulting in fewer operating hours on the engine.
While the previous discussion has concentrated on the example of a twin engine aircraft, it is apparent that the airflow and pressure regulation problem will be more severe on aircraft having more than two engines. For example, if a four engine aircraft has one pressure regulating valve supplying air at higher pressure than the other three, it is possible for that pressure regulating valve to swamp out the other three regulating valves, thereby resulting in a single engine to supply all the bleed air utilized by the aircraft. The results in this case are an unacceptably serious reduction in both fuel economy and engine wear characteristics.
Therefore, it has been desirable to have a system that can equalize the bleed air extraction from each engine, since such a system would result in better aircraft economy and a lower degree of mechanical wear in the engine. Such a system will likely pay for itself in a relatively short operating time, and therefore represents a worthwhile improvement to any bleed air control system.
A system and method for an engine bleed flow-sharing control system is disclosed. For a multi-engine bleed system, one of the engines is selected as the master channel such that the pressure at the inlet of the systems down stream receiving the bleed air is controlled to achieve a desirable inlet pressure range. To slave the other engines"" airflow control channels, the airflow rate is also measured in the master channel and the measured master airflow rate is used as the airflow setpoint for the slave channels. The difference between the airflow setpoint and the airflow rate in the slave channel is applied to control the pressure, or a valve opening area, of that slave channel.
In another embodiment of the present invention, the pressure within the master control channel is controlled by applying the difference between a pressure setpoint and the pressure in the master control channel. Similarly, to slave the other engines"" airflow control channels, the airflow rate is also measured in the master channel and the measured master airflow rate is used as the airflow setpoint for the slave channels. The difference between the airflow setpoint and the airflow rate in the slave channel is applied to control the pressure, or a valve opening area, of that slave channel.